In recent years, methods of using high energy ion implantation to create high concentration dopant regions deep within silicon semiconductor substrates have come into use. In such cases since secondary defect are created by ion implantation, beyond a certain threshold value of dopant concentration from ion implantation, it is difficult to achieve complete restoration of crystallinity even by annealing to restore the implantation damage. As an example of defects arising from high energy ion implantation, see Extended Abstracts of the 20th Conference on Solid State Devices and Materials, Tokyo, p 97-100 (1988).
FIG. 7 shows a prior art method of fabricating semiconductor devices using the ion implantation method. In FIG. 7(a), oxide film 72 is formed above CZ silicon substrate 71. Following this, in FIG. 7(b), dopant layer 75 is formed within silicon substrate 71 by ion beam 73 (Baron ion implantation). In this case, at roughly the same position as the dopant layer 75, a layer 74 damaged by ion implantation is formed, and at the surface and at the substrate sides undamaged fully crystalline layers 76 and 77 are formed.
The profile of the dopant atoms of this dopant layer 75 is a near Gaussian distribution having the peak concentration of dopant atoms at a depth position roughly at the center of the dopant layer 75. In other words, the dopant profile immediately following implantation is centered about its peak concentration with roughly symmetrical dopant profile tails in the depth direction (up and down). Similarly, there are also dopant profile tails following a certain distribution in the horizontal direction (front, back, left and right).
At the tips of these tails, normally, for example, a point where this conforms to the dopant concentration of the background semiconductor substrate is selected for the sake of convenience; a representative dopant concentration of the background is 1.times.10.sup.15 to 1.times.10.sup.16 cm.sup.-3.
Moreover, in FIG. 7(c), the said substrate 71 is subjected to annealing for the purpose of activating the implanted dopant and to recover the implantation damage. Through this annealing, the implantation damage within the substrate crystals become fully crystallized bidirectionally from the surface fully crystalline layer 76 and the substrate fully crystalline layer 77. At this time, the dopant profile following annealing has its peak concentration decreased due to diffusion and the tails of the dopant are spread up and down in the depth direction (similarly front, back, left and right horizontally).
Nevertheless, during the said annealing process, the deformation from the damage caused by implantation exceeding a certain amount of ion implantation (for example a dosage around 1.times.10.sup.14 cm.sup.-2) will cause a secondary defect 78 enclosed within the dopant layer 75 even after the annealing. The extreme difficulty of achieving restoration of the implantation induced secondary defect 78, once it is formed, is a known matter.
In clearly explaining such matters, the definitions of terms used for high energy implantation as used in the subject invention will be defined as follows: high energy implantation is that form of ion implantation wherein the peak of dopant concentration in the dopant layer formed by ion implantation to a monocrystalline semiconductor substrate, containing a damaged layer, is located within the semiconductor substrate, and where after the usual annealing (diffusion) the tails of this dopant layer (surface side of the semiconductor substrate) form a dopant layer profile which does not reach the surface of the semiconductor substrate. Or, even if this tail had reached the surface of the substrate, if the dopant concentration of the dopant layer's tail at the semiconductor substrate surface was, for instance, no more than around 20% of the substrate dopant concentration and did not markedly affect the characteristics of the device formed on the substrate surface, it should be considered as an object of the subject invention's high energy ion implantation.
Technically, the definition of high energy ion implantation is ion implantation in an energy region dominated by energy loss of ions due to the inelastic collision (electron energy loss) between implanted ions and electrons, while low energy ion implantation is ion implantation in an energy region dominated by energy loss of ions due to elastic collision (nuclear energy loss) between implanted ions and target atoms comprising the semiconductor. The threshold energy, as used in high energy ion implantation, where the electron energy loss becomes dominant over the nuclear energy loss is a value, for example, in the case of silicon semiconductors of around 17 KeV for boron (B) and 140 KeV for phosphorus (P). However, according to the above mentioned definition of high energy implantation terms, it is usual for the lower limiting values for high energy ion implantation to be more than several times greater than the technically defined threshold value.
Also, as a measure to reduce ion implantation induced secondary defects, ion implantation to silicon semiconductor substrates using FZ substrates had been attempted, and it has been reported that the density of ion implantation secondary defects, believed to be dependent on oxygen within the semiconductor substrate, has been reduced. For example, see Extended Abstracts of the 20th Conference on Solid State Devices and Materials, Tokyo, p. 97-100 (1988).
As related above, when using high energy ion implantation to create deep dopant layers in semiconductor substrates, even if a process of recrystallization through annealing after ion implantation is performed, implantation damage above a certain amount will become remaining deformation and bring about implantation secondary defects.
When using high energy ion implantation to form buried collectors for bipolar devices or well structures for CMOS devices, secondary defects caused by this sort of implantation damage will lead to junction leakage current, etc., and will adversely affect device characteristics.
It is known in prior art that in the so-called SIMOS structure fabrication of oxygen ion implantation of semiconductor substrate for device isolation (dielectric isolation), when forming planar silicon oxide region (dielectric) through the ion implantation of desired oxygen concentration, the generation of secondary crystal defects in the depth direction (up and down) of the single crystal region can be reduced by conducting a plurality of oxygen ion implantation and annealing. For example, see 5th International Workshop of Future Electron Devices--Three Dimensional Integration, Miyagi-Zao, p. 61-67 (1988).
In such a case, it is characteristic that the ion implanted region (oxydized region) loses semiconductivity and becomes a dielectric, and that the main problem is the secondary defects in the single crystal region adjacent to the ion implanted region. In this manner the subject of SIMOX is clearly different from a material standpoint both qualitatively and quantitatively from the subject of this invention which is to control the growth of defects within or peripheral to a semiconductive dopant layer formed by high energy ion implantation.
In SIMOX, it was a premise that in a position greater than a certain depth a planar silicon oxide region (dielectric) is formed continuously, and in principle a tertiary defect in the lateral direction within the non-crystalline oxide (ion implanted region) did not exist so that there was no need to take into consideration the generation of three-dimensional defects.
Nevertheless, unlike the case of SIMOX which unselectively forms planar regions of oxide film which becomes a dielectric, when using the technique of high energy dopant implantation of such as boron (B), phosphorus (P) and arsenic (As) into semiconductor substrate to selectively form devices buried dopant layers, the secondary defects extending in the depth direction (up and down) or planar side direction (left and right) become the cause of leakage current generation so that this is an important subject of study.